Antishrink treatment for wood



Patented Sept. 22, 1942 UNITED ..STATES PATENT OFFICE Henry A. Wallace, as Secretary of Agriculture of the United States of America, and his successors in office No Drawing. Application July 29, 1939,

Serial No. 287,308

2 Claims. (Cl. 21-9) (Granted under the act of March 3, 1883, as amended April 30, 1 928; 370 O. G. 757) This application is made under the act of March 3, 1883, as amended by the act of April 30, 1928, and the invention herein described and claimed, if patented, may be manufactured and used by or for the Government of the United States of America for governmental purposes, without the payment to me of any royalty thereon;

My invention relates to a process for permanently reducing the swelling and/or shrinking of wood, by subjecting kiln-dried, thoroughly air-dried, or chemically seasoned, wood to the action of heat (between 500 F. and 600 F.) for a short period of time minutes up to 2 hours, depending upon the temperature and size of the material), mider conditions which, practically eliminate surface oxidation, via, by immersing the wood in a suitable liquid bath comprising, for example, either molten metal; heated oil; or a fused salt.

The principal object of my invention is to permanently reduce the swelling and/or shrink= ing of wood.

Another object of my invention is to eliminate the remaining moisture in wood that can not be removed by any of the well-known commercial seasoning processes.

Still another object of my invention is to in crease the hardness of the wood. I

Still another object of my invention is to decompose chemicals, within the wood structure,

that have been employed in the seasoning of wood, so as to eliminate moisture condensation on the surface of the wood, when it is subjected to high relative humidities.

It has been known that heating of wood in an oven at normal oven-drying temperatures (220 F.) for several days reduces the subsequent swelling and/or shrinking to a slight extent. Heating wood to 250 F. or to 300 F. in dry air, or superheated steam, for periods of time ranging from half a day. to a week have been shown to give somewhat greater antishrink efliciencies. When the wood was heated to 300 F. for a week, fair antishrink efliciencies were obtained, but the degrade of the wood was excessive, undoubtedly clue to continued oxidation over this long period of time, or to hydrolysis from steaming. These observations have not led to commercial development, due to the degrade accompanying the attainment of a worthwhile antishrink efficiency, and the fact that heating of wood in a kiln to these temperatures for such a long period of time is both ineflicient and expensive.

My invention not only imparts a much greater mental oxidation of the woodby eliminating all oxygen from the system, with the exception of the small amount within the coarse capillary structure of the wood. Heating times are reduced from days to minutes or, at most, to a few hours, due to the higher temperature employed and the more rapid and efllcient transfer of heat to the wood by the use of a liquid heating media. I have found that the maximum antishrink efiiciency, with an accompanying minimum degrade, is imparted to wood when it is subjected to a heat treatment in a molten solder bath below or at the temperature at which destructive distillation begins. If the time of treating is sufficiently short, this temperature may be slightly exceeded.

My process is directed to the elimination of water of constitution from wood.

I have also found that subjecting wet wood, to high temperatures not only causes a largedegrade of the wood, but also defeats the purpose of the treatment by not reducing the swelling and/or shrinking of the wood. For this reason, I recommend that the wood be substantially dry before my heat treatment is applied.

The well-known methods of seasoning wood by heat treatment in a liquid media are not to be confused with my present invention, for the reason that I employ much higher temperatures; wood that -is substantially dry, and also, considerably shorter periods of time.

Specifically, my process will impart antishrink efficiencies as high as 50 percent to wood, without seriously reducing its mechanical properties. I have found that subjecting flat-sawn white pine specimens inch in thickness to a heat treatment in a molten solder bath at 500 F. for 1 hour gives a permanent reduction of the subsequent swelling and/or shrinking amounting to one-half that of untreated wood under the same moisture conditions. Thesame antishrink efficiency can be obtained at 550 F. by subjecting the wood to a heat treatment for 20 minutes;

and, at 600 F. by subjecting the wood to a heat treatment for only 10 minutes. Under these conditions, I have found that the white pine is darkened to about the color of walnut.

I have also found that this particular heat treatment appreciably increases the hardness of the wood.

The following examples illustrate the commercial possibilities of applying my process for permanently reducing the swelling and/or shrinking of wood. As these examples give only afew of a number of possible ways of carrying out my process, I do not wish to limit myself to the specific procedure, or type of equipment, but to the optimum temperatures and times of treatment in liquid media. that has for is purpose the accomplishment of the objective of my invention herein described.

Example 1.Take boards of substantially 1- inch thickness that have been previously kiln dried to an average moisture content of about 8 percent, and feed into a trough-shaped refractory lined bath containing a heated liquid, such as,

for example, a molten metal, such as, a low-fusion solder, or a fused salt, such as a mixture 01 sodium and potassium acetate. Hold the wood beneath the liquid surface and the while convey it from one end of the bath to the other under metal rolls conveniently spaced along the length of the bath and beneath the surface of the molten metal. I recommend, in order to provide thermal insulation, for example, that the bath be built in the ground, and that it be internally heated preferably by means of a low voltage-high amperage current, and to maintain the temperature at 550 F. Feed the'wood into the bath at such a rate that it is treated for half an hour. Pass the wood coming from the bath through any scraping or scrubbing device so as to remove any adhering metal, or fused salt, together with any slight surface char. Recover the metal or salt from the scrapings by burning when metal is used, or extraction and evaporation when salt is used.

It will be noted that the foregoing example is directed to the treatment of l-inch wood at 550 F. for hour. I may treat this same size wood at substantially 500 F, for substantially 1% hours, or atsubstantially 600 F. for substantially 15 minutes, and attain the same results as are obtained in Example 1.

It is to be understood that when thicker wood is treated, either correspondingly higher temperatures, or longer times, should be employed. And, when thinner woods, even down to veneer thicknesses, are treated, the converse is true.

Erample 2.Employ the same technique as in Example 1, with the exception of the bath medium and the means of heating.

In this example, I may employ a heat-resistant to diffuse part way into the wood, the degrade resulting from the subsequent seasoning is markedly reduced, because of the reduction in the tendency for the surface of the wood to shrink as a result of the presence of the hygroscopic chemical. Wood seasoned in this way has one serious shortcoming; namely, that the surface will become wet as a result of condensation of moisture from the air within the surface fibers of the wood under high relative humidity conditions, because of the hygroscopicity of the contained chemical. After the chemical has served its purpose in minimizing seasoning degrade, it would be desirable to convert it to a less hygroscopic chemical within the wood structure. This can be accomplished in the case where urea has been used as the seasoning chemical by subjecting the wood to a heat treatment according to my process hereinabove described. The urea will be completely decomposed to biuret and related compounds, together with ammonia. The biuret is a crystalline compound with only a small water solubility. Because of its extremely low hygroscopicity, it will not tend to cause moisture condensation on the wood as is the case for urea. The ammonia gas will be lost from the wood, together with the water of constitution of the wood during the heating process.

oil medium, such as, for example, hydrogenated oil, or high-boiling petroleum oils which may be heated internally by means of an electric resistance coil.

The temperature and time of treatment are the same as in Example 1,

The illustration following Example 1 as to thickness, temperature, and time of treatment are equally applicable for this example.

When the bath is to be heated by external means, such as gas jets, I recommend that the bath receptacle be made of metal.

The foregoing heat treatments of wood can serve a double purpose when the wood has been previously seasoned by a chemical seasoning method, for example, by the use of a thermally alterable substance, such as urea. When green wood is soaked in a concentrated solution of a relatively hygroscopic chemical, such as urea, for a sufficient period of time to allow the chemical I have also found that white'pine that has been seasoned by a chemical seasoning method with urea gives, on subjecting it to the heat treatment in the molten solder bath, a permanent reduction of the swelling and/or shrinking of the wood of the same order of magnitude as is obtained by subjecting dry wood which does not contain chemical seasoning agents to the heat treatment under the same conditions. Further, the heat-treated chemically seasoned wood does not become wet. on the surface as in the case of the wood chemically seasoned with urea, but not subjected to the heat-treating process, when it is stored in a room held at percent relative humidity.

It is to be understood that to practice my invention, no particular type of structure is required, and that any of the well-known baths, electrical hookup, or heating devices now on the market may be utilized without departing from the spirit of my invention.

Having thus described my invention, what I claim for Letters Patent is:

1. A method for permanently reducing the swelling and shrinking of wood, which comprises continuously passing dry wood beneath the surface of a fused salt consisting of a mixture of sodium and potassium acetates that is heated to substantially 550 F., at such a rate that l-inch thick wood is immersed for substantially hour,

the while excluding air from the wood; and,

thence removing any adhering salt and surface char present.

2. A method for permanently reducing the swelling and shrinking of wood, which comprises continuously passing dry wood beneath the surface of a fused salt consisting of a mixture of sodium and potassium acetates that is heated to substantially 600 F., at such a rate that l-inch thick wood is immersed for substantially 15 minutes, the while excluding air from the wood; and, thence removing any adhering salt and surface char present.

ALFRED J. STAMM. 

